1. Field of the Invention
The present invention relates to an injection molding machine that has a screw equipped with a check ring and is configured for detection of the state of wear of the check ring, screw, inner wall of the barrel, and so on.
2. Description of the Related Art
An injection molding machine stores molten resin in a front space inside the barrel and injects the molten resin into a mold by forward motion of a screw or plunger. In an injection molding machine of the in-line screw type, the screw is rotated, the resin is melted, the molten resin is fed forward to the space in front of the screw while the pressure of the resin moves the screw backward, the quantity of resin is thereby measured, and the screw is then moved forward to inject the molten resin into the mold. In this type of injection machine, the screw is equipped with a check ring near its front end to prevent backflow of resin during injection.
FIG. 1 illustrates an exemplary conventional check ring mechanism. The screw 1, which is inserted into the barrel 7, has a screw head 2, a check ring 3, movable in the axial direction of the screw 1, disposed in a section with a reduced diameter between the screw head 2 and the main body of the screw 1, and a check seat 4, disposed adjacent to the main body, that is brought into tight contact with the check ring 3 to close the resin path.
In the metering process, as the screw 1 rotates, resin pellets 8 are supplied from the rear end of the screw 1 and melted by both the shearing heat generated by rotation of the screw 1 and heat from a heater disposed outside the barrel 7 in which the screw 1 is inserted. The molten resin raises the resin pressure behind the check ring 3, producing a force pressing the check ring 3 forward. When the check ring 3 is pressed forward, the resin in the groove 6 behind the check ring 3 flows through a gap between the check ring 3 and the portion of reduced diameter into the space in front of the check ring 3, raising the pressure within the barrel 7 in front of the screw head 2.
When the resin pressure in front of the check ring 3 exceeds a predetermined pressure (the back pressure), the screw 1 is pressed backward to reduce the pressure in front of the check ring 3. The screw 1 continues to rotate, however, keeping the pressure behind the check ring 3 higher than the pressure in front of the check ring 3, so that the molten resin is continuously fed forward into the space in front of check ring 3. When the screw 1 has retreated to a predetermined distance (the measurement position), the rotation of the screw 1 is stopped and the metering process ends.
Next, the injection process starts. The screw 1 moves forward (from right to left in FIG. 1) to inject the resin into the mold. The resin pressure in front of the screw head 2 is thereby raised, so the check ring 3 is pressed backward and brought into tight contact with the check seat 4, closing the resin path and thus preventing backflow of the molten resin in the direction opposite to the advance of the screw.
Resin backflow from the front to the back of the check ring 3 occurs after injection starts and continues until the check ring 3 closes the resin path. This backflow occurs both through the resin path and through the gap between the outer circumference of the check ring 3 and the inner circumference of the barrel 7. When the injection molding machine is used over a long period of time, the check ring 3 and barrel 7 become worn and the rate of backflow changes accordingly. The backflow rate affects the amount of resin filled into the mold and thus the quality of the molded article.
Various methods of detecting the amount of leaking resin (the backflow rate) are known. For example, the backflow rate may be detected on the basis of the distance or speed of forward movement of the screw during hold pressure to prevent molding faults or detect wear or other problems with the check ring (See Japanese Patent Applications Laid-Open No. 62-3916 and No. 1-281912).
When the outer diameter of the check ring is reduced due to wear or the inner diameter of the barrel is enlarged due to wear, the backflow rate increases as described above. Accordingly, the state of wear of the check ring and barrel can be estimated from the backflow rate. In the inventions disclosed in the above Patent Documents, in which the backflow rate is detected on the basis of the distance or speed of forward movement of the screw during hold pressure, it would appear that the state of wear of the check ring and barrel could be estimated from the detected backflow rate. Unfortunately, the molded article shrinks during hold pressure, causing the screw to move forward. The above methods can not determine whether the screw has moved forward due to backflow or due to shrinkage of the molded article, so they cannot detect the exact backflow rate, and cannot accurately estimate the state of wear of the check ring and barrel.
In other known techniques, a resin flow stopping means is provided between the mold gate section and a nozzle section to stop the resin flow and thus apply a predetermined pressure to enable detection of the backflow rate on the basis of the distance by which the screw moves forward (Japanese Patent Applications Laid-Open No. 4-28519 and No. 4-263917).
In the inventions disclosed in the above Patent Documents, the screw does not move forward due to shrinkage of the molded article, but a special mechanism is required to stop the resin flow.
If resin backflow occurs during forward movement of the screw 1 as shown in FIG. 1, the flight 5 of the screw 1 is pressed backward by a force corresponding to the resin pressure (injection pressure) This force F is composed of a force Fx acting in the axial direction of the screw (pressing the screw backward) and a force Fθ acting in the screw rotational direction (turning the screw in the direction opposite to the direction during metering). Thus, the backflow applies a screw torque Fθ corresponding to the injection pressure to the screw 1. In another known method, a check ring abnormality is detected when the screw torque Fθ exceeds a predetermined value (See Japanese Patent Application Laid-Open No. 1-168421).
In the invention disclosed in the above Patent Document, wear and other problems of the check ring can be detected on the basis of the screw torque due to resin backflow, but it is not possible to detect the current degree of wear of the check ring before a problem occurs or to predict the remaining life of the check ring from the current degree of wear.